Field of the Invention
This invention relates to orthodontic appliances and in particular, to orthodontic brackets having a curved archwire slot that is round in cross section.
Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Orthodontic treatments for the repositioning of teeth employ a combination of appliances that are affixed to the teeth and archwires, which are coupled to the appliances. By placing the correct amount of tension on the archwires, the orthodontist is able to manipulate the position and orientation of a patient's teeth.
Orthodontic appliances, also known as “brackets” each possess an archwire slot, which accommodates an archwire. The archwire is a piece of stiff wire that has a curve that approximates the arc of the teeth in a human mouth. That is, from the rear molars of the left side of the mouth to the rear molars on the right side of the mouth, there is a natural curve that forms an arc of about 180 degrees. The arc is not circular but U-shaped. At the distal ends, the arc barely has any curve at all and is mostly straight. As the arc approaches the front teeth however, its curve becomes more pronounced.
To properly adjust the tension of the archwire, it is necessary that the interaction of the archwire in each archwire slot be as free from extraneous forces as possible. The torque that a properly adjusted archwire places on teeth acts in an inward/outward direction, normal to the surface of the tooth. Forces that act in a side-to-side direction, that is, along the arc of the archwire itself, are undesirable. Such forces can be present, however, because the ends of the archwire slots have edges that contact the archwire and act as a resistance to the movement of the archwire in the slot. This can be the case in locations in the mouth where the curve of the archwire is most pronounced. In some cases, the archwire rests almost entirely across opposite edges of the slot, which results in an unacceptable amount of friction as those edges dig in to the wire. Tightening the archwire then leads to moments with forces directed in undesirable directions.
Archwires may be either round or rectangular in cross section. A rectangular archwire gives more control to the orthodontist, but also presents problems. Rectangular archwires in rectangular slots produce friction. Each surface in the slot is a bearing surface that can produce friction and retard desired application of forces. In certain phases of the treatment, it may be advantageous to use one type versus the other. For example, the end phases of treatment frequently call for the use of a round archwire. Round archwires are chosen for this phase because there is less friction between a round wire and an archwire slot. However, archwire slots are made rectangular to accommodate rectangular or square archwires. Round archwires are commonly used in rectangular or square slots but this arrangement is less desirable than it could be. There is still friction produced by the round archwire engaging the straight walls of the slot. Additionally, round archwires do not torque well in rectangular slots because of the lack of contact between the wire and the walls of the slot.
Typically, orthodontic brackets have profiles that cause irritation of the inner lip surfaces of a patient because the bracket is rectangular in cross section and its edges protrude too far above the top of the tooth. Typical orthodontic bracket designs are shown in various U.S. patents such as Wildman et al. (U.S. Pat. No. 5,613,850), Damon (U.S. Pat. No. 5,466,151) and in published application no. US2004/0072117 to Farzin-Nia et al. (now U.S. Pat. No. 7,419,375).